1. Field of the Invention
The present invention relates generally to non-destructive evaluation (NDE) of materials. The present invention relates more specifically to a scanning technique for ultrasonic testing of materials that utilizes a realtime means for addressing loss of transducer contact with the material being evaluated.
2. Description of the Prior Art
One of the most frequently utilized techniques for the non-destructive evaluation of materials is the analysis and testing of the material through a systematic scan across the surface of the material with an ultrasonic transducer. Typically, this involves the use of a transducer that both transmits a generated ultrasonic signal and receives reflected and generated signals back from irregularities or anomalies hidden within the subject material. Appropriate interpretation of the received signals can aid in a description of the size, structure, distance, shape, and location of faults, cracks, or other anomalies.
The ability to accurately interpret information contained within the reflected and generated signals by the use of ultrasonic analyzing equipment requires that some baseline background signal, to some degree consistent over time, be established. If outside factors enter into the characteristics of the reflected and generated signals, then the interpretation and isolation of relevant data within the signals becomes quite difficult.
One such outside factor that can complicate the return signal is the occurrence of what is referred to as a "lift off" of the ultrasonic transducer from the surface of the material being tested. This lift off is simply the partial or complete loss of direct contact between the ultrasonic transducer and the material during scanning. Loss of transducer contact may be due to imperfections in the method of placing the transducer against the material, or may simply be the result of irregularities in the surface of the material. Lift off may, therefore, be caused by the mechanical positioning system that moves the transducer about the material, as well as by the properties of the material itself. Both of these factors which effect lift off will, of course, be position dependent. The mechanical loading will vary with the orientation of the positioning system and the structure of the specific positioning mechanism. The surface of the material will vary from point to point as a function of its manufacture and/or its wear over time.
Ultrasonic waves travel as longitudinal or shear waves in elastic materials. The speed of the ultrasonic wave depends upon the elasticity and density of the transmitting medium. Therefore, different mediums will transmit ultrasonic waves at rates that are characteristic of the substance.
Ultrasonic testing is possible because ultrasonic waves travel at different speeds through different mediums. By transmitting and receiving ultrasonic waves and tracking the time period over which these transmissions and receptions are made, a great deal can be understood about the medium through which the ultrasonic wave is traveling. Where the medium changes, such as where an anomaly or inclusion exists within a substance, the direction and speed of the ultrasonic wave is altered in a way that is indicative of some characteristic of the anomaly or inclusion. An ultrasonic transducer may be used to transmit ultrasonic waves into the material to be tested, and the reflected signal may be analyzed so as to detect and identify an irregularity within the material.
If the transducer is not in direct contact with the material to be tested, the first change in medium that the transmitted signal encounters is the interface between the transducer and the air space above the specimen. This interface will, therefore, create the first reflected and/or generated signal that is received back by the transducer. It is possible to misinterpret this received signal as indicative of some property of the specimen, rather than merely the occurrence of a lift off event. Since ultrasonic waves do not enter the specimen during the lift off event, irregularities in the specimen cannot be detected.
The loss of contact between the material and an ultrasonic transducer can result in a large localized change in the acoustic impedance seen by the analysis instrumentation. This change provides a significant detectable signal that must be distinguished from other anomaly sourced signals in order to be properly identified as resulting from a lift off event.
Previous attempts to address the problem of transducer lift off have been concerned primarily with minimizing its occurrence. Efforts to prevent lift off from the test material's surface, have attempted to maintain transducer contact by way of devices that adjust the transducer for irregularities in the surface, or by utilizing transducers whose transmitting and receiving face is flexible and/or configurable to the surface of the material. Unfortunately, these methods assume that certain expected irregularities will be encountered in the material's surface or that some normal inaccuracies in the mechanical positioning system used to scan the transducer across the surface will occur. None of the attempts thus far to consider the effect of lift off provide any realtime sensing or analysis of the phenomena, and are therefore subject to sometimes gross inaccuracies in handling signals that result from such lift off.
There have been some attempts, as mentioned above, to provide mechanisms whereby ultrasonic transducers may be accurately positioned and monitored with respect to the surface of the test material. U.S. Pat. No. 4,041,379, issued to Karlsson, discloses such an apparatus, wherein the inspecting ultrasonic transducers are held by means of pressure and tension members which are carried by a common support arm. The position, pressure, and tension of the transducer are independently controllable, and allow each transducer to be established with a predetermined force against the material surface.
U.S. Pat. No. 3,086,390, issued to Brown, describes an ultrasonic system intended for use in the medical field that utilizes an assembly of springs and weights to control the position of ultrasonic transducers on the non-planar surface of the human body. This system is designed to maintain a consistent contact with the skin of the patient undergoing analysis, despite the normal irregularities in the skin surface.
U.S. Pat. No. 4,043,185, issued to Siebert, discloses an ultrasonic transmitter and receiver that is supported by a device adapted to direct the transducer towards the surface of the material with a specific force, or to elevate the transducer in a manner that reduces the force against the material being tested.
Some designs such as that disclosed in Soviet Union Patent No. 1221594A control a nominal gap between the ultrasonic transducer and the test material so that the effect of surface irregularities is reduced to some extent.
Attempting to address the same problem but compensating for surface irregularities in a different way is U.S. Pat. No. 4,237,901, issued to Taenzer. The Taenzer patent describes an ultrasonic scanning system whose probe is a fluid filled transducer with a flexible surface that imparts a passive pressure against the material being tested. Irregularities in the surface of the material do not normally induce lift off, but only deform the face of the transducer. Unfortunately, this design is limited to certain specific applications, and is not appropriate for any broad range of materials testing concerns.
In general, the previous attempts to address the lift off problem in ultrasonic NDE testing have focussed on eliminating or at least reducing the number or degree of lift off events.